Tin nanoparticles and methodology for making same

ABSTRACT

A method of preparing tin (Sn) nanoparticles based on a bottom-up approach is provided. The method includes combining a first solution comprising Sn ions with a second solution comprising a reducing agent. After the combination, the Sn ions and the reducing agent undergo a reaction in which at least some of the Sn ions are reduced to Sn nanoparticles. The first solution comprises a tin salt dissolved in a solvent; the second solution comprises an alkali metal and naphthalene dissolved in a solvent; and the combined solution further comprises a capping agent that moderates a growth of aggregates of the Sn nanoparticles.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/397,250, filed on Mar. 3, 2009, which claims the benefit of priorityunder 35 U.S.C. §119 from U.S. Provisional Patent Application No.61/033,719, filed on Mar. 4, 2008, which is incorporated herein byreference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

FIELD

Embodiments of the present invention generally relate to tinnanoparticles and methodologies for synthesizing same. The disclosureenjoys particular utility in the manufacture of advanced, high capacitybatteries, e.g., reserve-activated batteries.

BACKGROUND

Tin (Sn) has a specific charge capacity of about 1,000 mAh/gm inelectrochemical applications, e.g., batteries. Current advanced,state-of-the-art Li-based batteries typically utilize carbon (C), i.e.,graphite, as a negative electrode material having a charge capacity ofabout 372 mAh/gm. A great amount of research is being performed with theaim of increasing the charge capacity of batteries, with tin and silicon(Si) currently being candidates for use in advanced lithium (Li)-basedbatteries.

If the charge capacities of materials presently available for Li-basedbatteries were increased by a factor of two or more, manufacture oflighter batteries or batteries with larger charge profiles, or withlonger service lifetimes would be facilitated. Such improvement inbattery capacities would have a great impact in a multitude oftechnologies ranging from hand-held electronic devices, e.g., mobilephones, to space-based systems and vehicles. However, several problemsare encountered with the use of Sn in Li-based batteries, includingvolume expansion due to intercalation of the Li. The result is animmediate and drastic reduction in the charge/discharge capacity aftercharging, as well as device failure.

Batteries containing materials comprising Li and Sn typically exhibit animmediate improvement vis-à-vis conventional Li-based batteries, but theimprovement dissipates with use, i.e., over several charge/dischargecycles. Improvements in the performance of such materials in batteryapplications have been obtained by reducing the sizes of individualgrains (i.e., particles). By reducing the particle size, the volumeexpansion upon intercalation can be diminished. Methods for reducing thesize of Sn particles typically rely on a “top-down” approach in whichlarger particles are made smaller. The most common of these methods aremechanical, e.g., use of a ball mill for reducing the size of largerparticles to achieve desired smaller particle sizes. Disadvantageously,however, such methods generally result in 0.5 micron or larger sizedmaterials, with poor-to-moderate control over specific particle size.Oftentimes, milling material abrades and contaminates the sample.

In view of the foregoing, there exists a need for improved approachesand methodologies for producing very small Sn particles, e.g.,nano-sized particles, of specified particle size or range of sizes,suited for use in the development of improved electrochemical powersources, e.g., advanced Li-based batteries.

SUMMARY

Embodiments described herein provide tin (Sn) nanoparticles suitable formaking advanced Li-based batteries by reducing tin ions with aparticularly effective reducing agent comprising an alkali metal andnaphthalene to form aggregates of Sn nanoparticles having a desirableand controlled size.

Certain embodiments provide a method for preparing tin (Sn)nanoparticles. The method comprises combining a first solutioncomprising Sn ions with a second solution comprising a reducing agent.After the combination, the Sn ions and the reducing agent undergo areaction in which at least some of the Sn ions are reduced to tinnanoparticles. The first solution can comprise a tin salt dissolved in asolvent. The second solution can comprise an alkali metal andnaphthalene dissolved in a solvent. The combined solution can furthercomprise a capping agent that moderates a growth of aggregates of thetin nanoparticles.

Certain embodiments provide a battery comprising one or more anodeelectrodes. The one or more anode electrodes can comprise aggregates oftin (Sn) nanoparticles coupled to conductive additives. The Snnanoparticle aggregates can be prepared by providing a mixture solutioncomprising at least Sn ions, an alkali metal, naphthalene, and a cappingagent, wherein the Sn ions and the alkali metal undergo a reaction inwhich at least some of the Sn ions are reduced to aggregates of Snnanoparticles.

It is to be understood that both the foregoing summary and the followingdetailed description are exemplary and explanatory and are intended toprovide further explanation of the embodiments as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1 is a flowchart illustrating an exemplary process for preparingtin (Sn) nanoparticles and aggregates thereof based on a bottom-upapproach according to certain disclosed embodiments.

FIG. 2 is a graph illustrating the results of X-ray photoelectronspectra (XPS) studies indicating ability of methodologies according tothe present disclosure to synthesize Sn nanoparticles or Snnanoparticles having a SnO₂ shell; and

FIG. 3A and FIG. 3B are scanning electron microscopy (SEM) and scanningtransmission electron microscopy (STEM) images illustrating large andsmall aggregates of Sn nanoparticles, respectively.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth to provide a full understanding of the disclosed and claimedembodiments. It will be apparent, however, to one ordinarily skilled inthe art that the embodiments may be practiced without some of thesespecific details. In other instances, well-known structures andtechniques have not been shown in detail to avoid unnecessarilyobscuring the disclosure. The word “exemplary” is used herein to mean“serving as an example, instance, or illustration.” Any embodiment ordesign described herein as “exemplary” is not necessarily to beconstrued as preferred or advantageous over other embodiments ordesigns.

Broadly stated, the present disclosure is based upon discovery that, incontradistinction to the prior “top-down” approach and methodology forproducing very small Sn particles, e.g., nano-sized particles, a“bottom-up” synthesis approach is advantageous for obtaining ultra-smallSn particles of specified particle size or range of sizes, especiallysuited for use in the development of improved electrochemical powersources, e.g., advanced Li-based batteries. Methodologies according tothe present disclosure are capable of synthesizing Sn nanoparticleshaving sizes as small as a few nanometers in diameter and up to severalhundreds of nanometers. Sn nanoparticles synthesized according to thepresently disclosed “bottom-up” methodology are smaller than those madeby conventional “top-down” methodology, e.g., ball milling. Further, the“bottom-up” approach facilitates tailoring of particle sizes forobtaining optimum electrochemical properties. In addition, the surfacecharacteristics of the nanoparticles may be adjusted, which feature isimportant for obtaining wettability of the particles with theelectrolyte solution—a requirement for good battery performance.Finally, according to the “bottom-up” approach, the surface morphologyof the nanoparticles, which dictates electrical conductivity andreactivity, can be tailored for specific applications.

Additional advantages and aspects of the present disclosure will becomereadily apparent to those skilled in the art from the following detaileddescription, wherein embodiments of the present methodology are shownand described, simply by way of illustration of the best mode currentlycontemplated for practicing the methodology described herein. As will bedescribed, the present disclosure is capable of other and differentembodiments, and its several details are susceptible of modification invarious obvious respects, all without departing from the spirit of thepresent disclosure. Accordingly, the drawings and description are to beregarded as illustrative in nature, and not as limitative.

More specifically, according to the “bottom-up” approach of the presentdisclosure, a compound of tin, e.g., a tin salt, is reduced to metallictin. FIG. 1 is a flowchart illustrating an exemplary process 100 forpreparing tin nanoparticles and aggregates thereof based on a bottom-upapproach. The process 100 begins at a state 101 in which a firstsolution comprising Sn ions is prepared by adding a tin salt to asolvent. Suitable tin salts include anhydrous and hydrated tinchlorides, specifically tin (II) chloride (SnCl₂) and tin (IV) chloride(SnCl₄). By way of illustration only, the following steps are describedwith respect to an exemplary synthetic route utilizing a tin salt. Inthe exemplary synthetic route, 20 mmol of SnCl₂.2H₂O and 20 mmolhydrobenzamide (tribenzaldiamine) were added to 20 mL of THF(tetrahydrofuran) and heated to about 60° C. under an argon (Ar)atmosphere. However, the bottom up approach of the present disclosure isnot limited to the exemplary synthetic route.

The process 100 proceeds to a state 102 in which a second solutioncomprising a reducing agent is prepared. In the exemplary syntheticroute, 45 mmol of elemental sodium (Na) and 48 mmol of naphthalene weredissolved in 50 mL of THF in a separate flask, also under an Aratmosphere. This procedure resulted in sodium naphthalide solution. Inthe reducing reaction to be described below, the naphthalene provides acatalyst or a capture agent which captures Na ions and allows them toefficiently release electrons, which electrons then reduce tin (e.g.,Sn⁺²) ions to synthesize Sn nanoparticles.

The process 100 proceeds to a state 103 in which the first solution iscombined with the second solution to initiate a chemical reaction. Inthe illustrated example, the combination was achieved by transferringthe sodium naphthalide solution, via a cannula, to the SnCl₂ solution,for initiating a reaction in which the Sn'² ions of the SnCl₂ arereduced to Sn. This reaction takes place in oxygen-free environment(using standard Schlenk techniques).

In this reaction, the hydrobenzamide serves as a “capping” agent whichmoderates the growth of aggregates of Sn nanoparticles. For example, thecapping agent, such as the hydrobenzamide, controls (e.g., forestalls,limits, retards) the aggregation of the Sn nanoparticles by attaching tothe Sn nanoparticles and providing a steric hindrance to restrict growthand prevent aggregation. In general, the size of the nanoparticles canbe determined at least in part by a combination of factors including:(1) the hydrobenzamide concentration; (2) the reaction temperature; and(3) the duration of reaction. In this reaction, the elemental sodium(Na) acts as the reducing agent for reducing the tin (II) chloride(SnCl₂) by means of the following reaction:

SnCl₂+2Na→Sn+2NaCl

the process 100 proceeds to a state 104 in which the reaction is allowedto proceed and be completed. Excess sodium is provided to ensure thatthe reaction proceeds to completion and to assist in removal of thehydrate, i.e., water, from tin salt (which may arise from an as providedhydrated salt or contamination). The duration of the reduction reactioncan be on the order of minutes to several hours, depending on variousfactors such as the desired median size of the Sn nanoparticles, theparticular capping agent used, and the reaction temperatures.

The process 100 proceeds to a state 105 in which solvent is removed anddry samples of synthesized Sn nanoparticles are obtained after apurification process. Such a purification process to obtain synthesizednanoparticles is well known in the art. In an exemplary process, asolution containing the synthesized nanoparticles are separated into apolar phase and a non-polar (organic) phase. The nanoparticles can beobtained, e.g., collected, from the non-polar phase of the solution. Theprocess ends at state 106.

While the process 100 illustrates an exemplary bottom-up methodology, itshall be appreciated by those skilled in the art in view of the presentdisclosure that many alternatives to the illustrated process arepossible. For example, the preparation of the second solution (102) canoccur prior to or concurrently as the preparation of the first solution(101). In addition, stoichiometries (i.e.,hydrobenzamide/sodium/naphthalene ratios) other than those described inthe foregoing illustrative example may also be utilized.

In an alternative experimental embodiment, a reaction described by

SnCl₄+4Na→Sn+4NaCl

may be performed with the tin (IV) chloride (SnCl₄), as follows: A firstsolution comprising 1 molar equivalent of SnCl₄ and 1 molar equivalentof capping agent (trioctylphosphine) dissolved in THF is prepared. Asecond solution comprising 4:3 molar equivalent of sodium:naphthalenedissolved in THF is prepared. The second solution is added to the firstsolution, and the mixture solution is stirred at 60° C. for 1 hr. Theanhydrous material requires extreme care and must be handled in anoxygen-free environment. This reaction produced aggregates of Snnanoparticles having a median size of about 20 nm, as determined by XPSstudies to be described below with respect to FIG. 2, and SEM and STEMstudies to be described below with respect to FIGS. 3A and 3B. As avariation on the above alternative experimental embodiment, the tin (IV)chloride can be added directly to the sodium naphthalene solutioninstead of preparing a separate solution comprising the SnCl₄.

In yet another alternative experimental embodiment, a first solutioncomprising 1 molar equivalent of SnCl₂×2H₂O mixed in THF with certainmolar equivalents of capping agent (e.g., 2 mol eq. triphenyl phosphine,1 mol eq. hydrobenzamide, etc.) effective to produce nanoparticleaggregates having a target median size is prepared. A second solutioncomprising a 5:3.5 (excess sodium—4.5:3 used also) molar equivalent ofsodium:naphthalene dissolved in THF is prepared. The second solution isthen added to the first solution, and the mixture solution is stirred ata temperature varying from room temp to 80° C. and for a reaction timethat can be varied from 10 minutes to 2 hrs. Common reaction time was 30min.

Furthermore, capping agents other than hydrobenzamide, such as, forexample and without limitation triphenylphosphine (TPP),trioctylphosphine (TOP), cetyltributylammonium bromide (CTAS),hexylamine, and butylamine may be utilized. Other suitable cappingagents may include different functional groups, such as, for example andwithout limitation, alcohols, thiols, and carboxylates. Additionally,while the illustrated reaction employed sodium as a reducing metallicagent, other metallic elements including alkali metals such as lithium(Li) and potassium (K) can be employed instead. For example in place ofthe sodium/napthalene combination as the reducing agent, a number ofother reducing agents, e.g., sodium borohydride and butyl lithium, maybe employed. Other usable solvents, depending upon the reducing agentand the desired end products, include glycol ethers, ethylenediamine,and water. Examples of the glycol ethers include glyme and triglyme.

Reducing agents employing naphthalene as an initiator (e.g., sodiumnaphthalide) were experimentally shown to be superior compared toreactions employing other types of reducing agents (e.g., sodiumborohydride and butyl lithium), in terms of the variety of cappingagents that can be used and the degree of control that can be exercisedover the size of the Sn nanoparticles by varying the concentration ofthe capping agents. These superior characteristics of reducing agentscomprising naphthalene molecules as the catalyst can be attributed tothe molecule's superior capability to capture the alkali metals andfacilitate the release electrons from the captured metals. The highreducing capability of the naphthalide solution allows one the abilityto efficiently reduce the variety of tin salts used for the disclosedreactions. The capping agent used for the reactions is designed tofunction as an inhibitor in the sense that it slows the reduction of theSn ions while also providing a protected surface to prevent aggregation.By controlling the reaction rate, one can take advantage of theexcellent reducing capacity offered by using a sodium solution (e.g.,sodium naphthalide).

X-ray photoelectron spectra (XPS) studies were performed on dry samplesin order to measure/determine the composition of the nanoparticles. FIG.2 is a graph 200 illustrating the results of X-ray photoelectron spectra(XPS) studies indicating ability of the methodology according to thepresent disclosure to synthesize Sn nanoparticles or Sn nanoparticleshaving a SnO₂ shell. The graph 200 includes two traces 210 and 220corresponding to XPS spectra of two different Sn nanoparticle samples.The trace 210 shows a binding energy peak 202 associated with Snnanoparticles, indicating that the corresponding sample is comprisedalmost exclusively of Sn nanoparticles.

On the other hand, the trace 220 shows a binding energy peak 201associated with Sn nanoparticles as well as a binding energy peak 203associated with tin oxide (SnO₂), indicating that the correspondingsample is comprised of Sn nanoparticles and one or more SnO₂ shells atleast partly surrounding the Sn nanoparticles. Such surrounding SnO₂shells can advantageously provide solid electrolyte interface (SEI)layers that can prevent active materials (e.g., Sn nanoparticles) frombeing transported from their current locations to separator materials.In one aspect, by selecting a suitable capping agent, it is possible toproduce Sn nanoparticles having SnO₂ shells (as evidenced by the trace220). For example, the use of capping agents such as hexylamine,butylamine, and trioctylphosphine, is conducive to the formation of theSnO₂ shells because the capping agents, while controlling theaggregation of the nanoparticles, have a decreased ability in protectingthe Sn nanoparticles from oxidation. As a result, the Sn nanoparticleslocated on the outer surface of a Sn aggregate become oxidized to form aSnO₂ shell surrounding the interior Sn nanoparticles.

Scanning electron microscopy (SEM) and scanning transmission electronmicroscopy (STEM) images, shown in FIG. 3A and FIG. 3B, illustrate theability of the presently disclosed methodology to form relatively largeaggregates of Sn nanoparticles, i.e., with diameters on the order of˜300 nm, that are composed of many smaller sized Sn nanoparticles, i.e.,with diameters on the order of ˜20 nm. For example, FIG. 3A shows a SEMimage 310 highlighting aggregated Sn nanoparticles 311 in which theaggregates are about 300 nm in diameter. The long fibrous structure 312is lacey carbon, which provides a support structure for the aggregatedSn nanoparticles 311. The sheet-like material 313 is the polymer used tosupport the lacey carbon 312. Conductive additives that can be used toprovide a conductive path between aggregates include carbonnanomaterials (e.g., nanotubes, nanorods, nanoplates, nanofibers, andthe like) and conductive graphite. FIG. 3B, on the other hand, shows aSTEM image 320 of smaller Sn nanoparticle aggregates 321 of about 20 nmin diameter. The light grey coating 323 surrounding the nanoparticles isa polymer material trapping the nanoparticles against the lacey carbon322.

The use of the disclosed methodology is shown to provide Snnanoparticles having a tighter size distribution as compared to thetop-down methodologies. For example, the sizes of the Sn nanoparticlessynthesized according to the disclosed methodology are shown to varywithin +/−5 nm. In comparison, the ball milling methods are known toproduce Sn particles having a wide range of sizes, ranging, e.g., fromunder 1 μm to several microns. In some cases, in order to obtainparticles having sizes less than, for example, 1 μm, a filtering may benecessary to remove larger sized particles.

In summary, the presently disclosed “bottom-up” methodology is wellsuited for use in forming improved anode electrodes of high capacityLi-based batteries, which anode electrodes may comprised of Snnanoparticles and conductive additives (e.g., carbon nanotubes) forincreasing charge capacity with reduced cell volume increase.Introduction of the Sn nanoparticles into the anode electrodes ofLi-based batteries increases the anode charge capacity from less thanabout 400 mAh/gm. to just under 1,000 mAh/gm. By contrast, “top-down”methodologies for forming Sn nanoparticles, e.g., ball milling, resultin larger particle sizes with little or poor control over particle sizeand size distribution.

The synthesis method according to the present disclosure readily andefficiently produces size controllable Sn nanoparticles with mediansizes ranging from ˜20 nm to several hundred nm via variation ofreaction parameters such as concentration of a capping agent, reactiontemperature, and reaction duration. In certain embodiments, thesynthesis method produced aggregates of Sn nanoparticles having a mediansize in the range of 20 nm to 100 nm. In other embodiments, thesynthesis method can produce Sn nanoparticles aggregates having a mediansize in excess of 100 nm, up to about 500 nm. In some embodiments, thesynthesis method produced Sn nanoparticle aggregates having sizesfalling within +/−5 nm from the median size. The nanoparticles arecolloidal in nature and very stable. Solvent removal results inaggregation into larger particles, which aggregates offer the potentialfor high charge capacity while retaining size and structure suitable forcoupling with (e.g., attaching to, fixed to, or surrounding) conductiveadditives (e.g., the carbon nanotubes) to form a composite material.U.S. Patent Application Ser. No. 61/099,342, filed on Sep. 23, 2008,which is incorporated by reference herein in its entirety, providesdetails on forming such a composite material for a particular batteryapplication.

In the previous description, numerous specific details are set forth,such as specific materials, structures, processes, etc., in order toprovide a better understanding of the present disclosure. However, thepresent disclosure can be practiced without resorting to the detailsspecifically set forth herein. In other instances, well-known processingtechniques and instrumentalities have not been described in order not tounnecessarily obscure the present disclosure.

Only the preferred embodiments of the present disclosure and but a fewexamples of its versatility are shown and described herein. It is to beunderstood that the present disclosure is capable of use in variousother combinations and environments and is susceptible of changes and/ormodifications within the scope of the inventive concept as expressedherein.

The foregoing description is provided to enable any person skilled inthe art to practice the various embodiments described herein. While theforegoing embodiments have been particularly described with reference tothe various figures and embodiments, it should be understood that theseare for illustration purposes only and should not be taken as limitingthe scope of the invention.

There may be many other ways to implement the invention. Variousfunctions and elements described herein may be partitioned differentlyfrom those shown without departing from the spirit and scope of theinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art in view of the present disclosure,and generic principles defined herein may be applied to otherembodiments. Thus, many changes and modifications may be made to theinvention, by one having ordinary skill in the art in view of thepresent disclosure, without departing from the spirit and scope of theinvention.

A reference to an element in the singular is not intended to mean “oneand only one” unless specifically stated, but rather “one or more.” Theterm “some” refers to one or more. Underlined and/or italicized headingsand subheadings are used for convenience only, do not limit theinvention, and are not referred to in connection with the interpretationof the description of the invention. All structural and functionalequivalents to the elements of the various embodiments of the inventiondescribed throughout this disclosure that are known or later come to beknown to those of ordinary skill in the art are expressly incorporatedherein by reference and intended to be encompassed by the invention.Moreover, nothing disclosed herein is intended to be dedicated to thepublic regardless of whether such disclosure is explicitly recited inthe above description.

1. A method comprising: forming a reaction mixture comprising a solutionof a tin salt, a reducing agent, and a capping agent; and reducing atleast a portion of the tin salt in the presence of the reducing agentand the capping agent to form aggregates of tin nanoparticles; whereinthe capping agent moderates a growth of the aggregates of tinnanoparticles, the capping agent comprising a functional group selectedfrom the group consisting of an amine, a phosphine, a thiol, and acarboxylate.
 2. The method of claim 1, wherein forming a reactionmixture comprises combining a first solution comprising the tin saltwith a second solution comprising the reducing agent.
 3. The method ofclaim 2, wherein the capping agent is present in the first solution. 4.The method of claim 1, wherein the reducing agent comprises an alkalimetal and naphthalene.
 5. The method of claim 1, wherein the reducingagent comprises sodium borohydride or butyl lithium.
 6. The method ofclaim 1, wherein the tin salt comprises tin (II) chloride or tin (IV)chloride.
 7. The method of claim 1, wherein the capping agent is presentin the reaction mixture before reducing the tin salt takes place.
 8. Themethod of claim 1, wherein the tin nanoparticles further comprise a tinoxide (SnO₂) shell at least partially surrounding the tin nanoparticles.9. The method of claim 1, wherein a median size of the tin nanoparticlesranges from about 20 nm to about 100 nm.
 10. The method of claim 1,wherein a median size of the tin nanoparticles ranges from about 100 nmto about 500 nm.
 11. A method comprising: forming a reaction mixturecomprising a solution of a tin salt, a reducing agent, and a cappingagent; reducing at least a portion of the tin salt in the presence ofthe reducing agent and the capping agent to form aggregates of tinnanoparticles; and allowing the tin nanoparticles to at least partiallyoxidize to form a tin oxide (SnO₂) shell at least partially surroundingthe tin nanoparticles; wherein the capping agent moderates a growth ofthe aggregates of tin nanoparticles, the capping agent comprising afunctional group selected from the group consisting of an amine, aphosphine, a thiol, and a carboxylate.
 12. The method of claim 11,wherein forming a reaction mixture comprises combining a first solutioncomprising the tin salt with a second solution comprising the reducingagent.
 13. The method of claim 12, wherein the capping agent is presentin the first solution.
 14. The method of claim 11, wherein the reducingagent comprises an alkali metal and naphthalene.
 15. The method of claim11, wherein the capping agent is present in the reaction mixture beforereducing the tin salt takes place.
 16. A battery comprising one or moreanode electrodes, the one or more anode electrodes comprising aggregatesof tin nanoparticles prepared by the method of claim
 1. 17. The batteryof claim 16, wherein the battery is a lithium battery.
 18. The batteryof claim 16, wherein the one or more anode electrodes further comprisecarbon nanotubes.
 19. The battery of claim 16, wherein the tinnanoparticles further comprise a tin oxide (SnO₂) shell at leastpartially surrounding the tin nanoparticles.
 20. The battery of claim16, wherein a median size of the tin nanoparticles ranges from about 20nm to about 100 nm.